High voltage ceramic capacitor assembly

ABSTRACT

A ceramic capacitor assembly for high voltages comprising a plurality of ceramic capacitor blocks disposed in stacked relation within a tubular insulating housing. A spacer ring of insulating material is associated with each block for maintaining the peripheries of the blocks radially spaced from the insulating housing by a predetermined minimum amount sufficient to effectively prevent ionization in the gaps between the peripheries of the blocks and the tubular housing. Each spacer ring is anchored to its associated ceramic block in such a manner that all portions of the spacer ring that are disposed radially outward of the periphery of the ceramic block are located in a position axially spaced from each ceramic block.

1 HIGH VOLTAGE CERAMIC CAPACITOR ASSEMBLY This invention relates to ahigh voltage ceramic capacitor assembly of the type shown and claimed inapplication Ser. No. 817,848-Nakata, filed Apr. 21, 1969., now U.S. Pat.No. 3,586,934, and assigned to the assignee of the present invention.

In that application, there is shown and claimed a capacitor assemblythat comprises a plurality of ceramic capacitor units stacked in axiallyaligned relationship and electrically connected in series. Eachcapacitor unit comprises a ceramic block having spaced-apart planarfaces covered with a conductive coating and a pair of end rings ofelectrical insulating material bonded to said conductive coatings. Theperiphery of the bonded capacitor unit is subjected to a grindingoperation that removes sufficient material to achieve precise alignmentbetween the outer peripheries of the ceramic block, the conductivecoatings, and the end rings. It is customary to include as part of suchan assembly a tubular housing of insulating materials surrounding thestacked capacitor units.

When such a housing is present around the capacitor units, there existsthe possibility that there will be small critical gas gaps between thecapacitor units and the tubular insulating housing. When a high voltageis applied to the capacitor stack,

these gas gaps can ionize and trigger a breakdown across the capacitorunits.

To prevent such critical gas gaps from being present, spacing means isneeded in order to maintain the capacitor units radially spaced form thetubular insulating housing by a distance greater than the critical gaplen An object of our invention is to construct and arrange the spacingmeans in such a manner that it can be of an inexpensive material and canbe easily and inexpensively incorporated into the capacitor assembly.

Another object is to construct and arrange the spacing means in such amanner that its presence causes no critical gas gaps to be introducedbetween the spacing means and the ceramic capacitor unit.

Still another object is to construct the capacitor assembly in such amanner that if the bond that is present between the capacitor unit andthe spacing device fails, the spacing device still remains in place toperform its desired spacing function.

In carrying out the invention in one form, we provide each capacitorunit with spacing means for maintaining the periphery of the ceramiccapacitor block radially spaced from wiched between two relativelystrong parts the tubular insulating housing by a predetermined minimumamount sufficient to effectively prevent ionization in the region of theceramic periphery. This spacing means comprises two spacer rings ofelectrical insulating material, each anchored to oneof the insulatingend rings of the ceramic capacitor unit. These spacer rings extendradially outward beyond the outer periphery of the ceramic block, andall portions of the spacing rings which are disposed radially outward ofthe periphery of the ceramic block are located in a position axiallyspaced from each of said ceramic blocks.

For a better understanding of the invention, reference may be had to thefollowing description taken in conjunction with the accompanyingdrawings, wherein;

FlG. l is a sectional view of a capacitor assembly embodying one form ofour invention.

FIG. 2 is a sectional view along the line 2-2 of FIG. 1.

FIG. 3 is a detailed sectional view illustrating a construction whichshould be avoided.

' Referring now to FIG. 1, the capacitor assembly is designated 9 andcomprises a plurality of ceramic capacitor units 10 stacked inend-to-end relationship along the central longitudinal axis 11 of thecapacitor assembly.

Each capacitor unit 10 comprises a cylindrical ceramic disc or block 12having a pair of opposed planar faces 13 extending transversely of thelongitudinal axis 11 and a cylindrical periphery 14 extending betweenthe planar faces generally parallel to the longitudinal axis 11. Eachplanar face 13 is covered by an end coating 15 of conductive materialbonded thereto. Preferably, this coating is a silver dispersion fusedintegrally onto the ceramic face 14. Each of the ceramic disks is madeof a ceramic material having a high dielectric strength and a highdielectric constant, such as, for example, barium titanate. A typicaldielectric constant is in the range of several hundred to severalthousand relative to that of air.

Also forming a part of each capacitor unit are two end rings 18 made ofan electrical insulating material such as steatite porcelain that hassubstantially the same coefficient of thermal expansion as the ceramicmaterial of the gap. Each of these end rings has a planar face on onesurface located adjacent the conductive coating 15. A suitable cement isrelied upon to adherently bond this face of the end ring 18 to theadjacent conductive coating 15.

Each of these capacitor units 10 is method disclosed and claimed in theNo. 817,848. In this respect, after the end rings 18 have been bonded tothe ceramic capacitor element 12, the resulting bonded unit is placed ina centerless grinding machine; and the periphery of the bonded unit isground until precise alignment is attained between the outer peripheryof the ceramic element 12, the outer periphery of the end rings 18, andthe outer periphery of the conductive coatings 15. Of particularimportance is the fact that afler such grinding the peripheral edge ofeach conductive coating 15 is located precisely flush with the finalperipheral surface 14 of the adjacent ceramic capacitor element and isin virtually perfect alignment with said peripheral surface.

The metal coating 15 is relatively fragile, and grinding does have atendency to produce tiny peripheral chips in this coating; but suchchips are avoided because the coating is sand- 12 and 18 in intimateengagement with the coating about the entire area being ground. Thepresence of the end rings 18 also serves to prevent chips fromdeveloping in the periphery of the ceramic capacitor element 12 adjacentthe conductive coating during the grinding operation.

After each unit 10 has been ground to provide a smooth periphery alongits entire length, the periphery of the unit is coated with a thin layeror film of dielectric material 20, typically having a thickness of about2 mils. A suitable material for this coating 20 is a silicone orpolyester varnish. The purpose of the thin dielectric coating is tosuppress field emission and micro-discharge initiated breakdowns fromthe edge of the metal coatings 15. Coverage of the insulating end rings18 by this thin coating 20 is not important except insofar as itfacilitates coverage of the outer peripheral edge of the juxtaposed endelectrodes 15. The relative dielectric constant of the coating istypically about 3 to 5.

For electrically connecting the ceramic capacitor units in series, aplurality of conductive buttons 24 are provided. One of these conductivebuttons 24 is located between each pair of ceramic capacitor units 10.Each of these conductive buttons is preferably of generally cylindricalform and has a pair of opposed planar faces which contact the metalcoatings 15 of the ceramic units 10 to provide a 'good electricalconnection between the button and the conductive coatings. Each button24 has an enlarged central portion 25 thereon, the purpose of which willsoon be explained.

For maintaining the capacitor units 10 and the buttons 24 in stacked,axially aligned relationship, a tubular housing 30 of insulatingmaterial is provided about these elements. At opposed ends of thetubular insulating housing 30, metallic terminal structures 32 and 34for the capacitor assembly are provided. These temiinal structures 32and 34 are respectively connected to the tubular insulating housing 30by suitable means such as threaded joints 35 and 36. These terminalvoltage power circuit by the top of the capacitor aspreferably made bythe aforesaid application Ser.

suitable means, (not shown). At sembly, a conductive button 38,preferably, but not necessarily, brazed to the top terminal structure32, bears against the conductive coating 15 on the top of the topceramic element 12. At the bottom of the capacitor assembly, aconductive button 44 bears against the conductive coating 15 on thebottom of the bottom ceramic element 12. A flexible conductiveconnection in the form of a compression spring 39 is provided betweenthe lower terminal structure 34 and the button 44 to carry currentbetween these parts. This spring bears at one end on terminal structure34 and at its other end on button 44. Contact between the buttons andtheir adjacent ceramic capacitor units is maintained by the compressionspring 39. Compression spring 39 provides an upward force which urgeseach internal element of the assembly upward into firm engagement withits adjacent element. It will therefore be apparent that the capacitorelements are electrically connected in series circuit relationshipbetween the terminal structures 32 and 34.

When a tubular housing such as 30 is present around the capacitor units,there exists the possibility that there will be small critical gasgapsbetween the capacitor units and the tubular housing. When a high voltageis applied to the capacitor assembly, these'gas gaps can ionize andtrigger a breakdown across the capacitor assembly. To prevent suchcritical gas gaps from being present, we provide spacing means thatserves to maintain the capacitor units radially spaced from the tubularhousing 30 by a distance greater than the critical gap length.

This spacing means comprises, for each capacitor unit 10, a pair ofspacer. rings 40 of electrical insulating material disposed at axiallyopposed ends of the unit. Each of these spacer rings 40 is bonded by asuitable adhesive at 41 to the outer face of an end ring 18. Each spacerring 40 projects radially outward beyond the outer periphery of theadjacent end ring 18 and has an outer periphery 42 fitting looselywithin tubular housing'30. If a capacitor unit 10 shifts laterally by asmall amount, this outer periphery 42 of its spacer ring will contactthe inner. periphery of tubular housing 30 to limit further lateralshifting. The diameter of each spacer ring issufficiently large to limitthe minimum spacing between the tubular housing 30 and the outerperiphery 14 of the ceramic capacitor element 12 to a distance greaterthan the critical gap length.

. Eachspacer ring 40 comprises a radially extending body portion and anaxially extending flange 43 which surrounds the adjacent end ring 18.One function of this flange 43 is to locateand center the spacer ring 40with respect to the capacitor unit. Another function is to prevent anysubstantial radial shifting of the capacitor unit 10 with respect to thespacer ring 40 should there be a failure of the bond between spacer ring40 and end ring 18. In this latter regard, it is to be noted that eachof the conductive buttons 24 has an enlargement or shoulder 25 thereonwhich will prevent axial movement'of the spacer ring 40 by more than adistance E should the bond at 41 fall. This distance E is shorter thanthe length of flange 43. Thus, even if the spacer ring moved axiallythrough distance E, the flange 43 would still be surrounding end ring 18and in a position to prevent substantial shifting of the capacitor unit10 with respect to the spacer ring 40.

The bond between each of the end rings 18 and its adjacent conductivecoating 15 must be a high quality bond in order to enable the end ringto function as above described to protect the conductive coating duringthe above-described grinding operation. For this reason, care is takento completely cover the interface between 18 and 15 with adhesive and tocure the adhesive by means of a baking operation. Far less care need be1 taken with the bond at 41 between spacer ring 40 and the end ring 18.Even if this latter bond should fail, spacer ring 40 can still performits intended function, as described hereinabove. Accordingly, the spacerrings are preferably applied after the capacitor unit is otherwisecomplete and without reliance upon baking to curethe adhesive at 41. Thespacer rings 40 should be of an electrical insulating material, but maybe of a very inexpensive material since their function is primarily amechanical one and the bond at 41 is not a crucial one. Since the bondat 41 is not crucial and requires no baking, it is not necessary toutilize for ring 40 a material whose coefiicient of thermal expansionclosely matches that of adjacent ring 18.

It is most important that the flange 43 on each of the spacer rings 40terminate along a line that is axially spaced from its ceramic block 12.No portion of the spacer ring 40, including flange 43, should extendinto axially overlapping relation with the periphery 14 of the ceramicblock 12. FIG. 3 has been included to show this overlapping conditionthat should be avoided. In this figure, flange 43a extends intooverlapping relation with periphery 14 of the ceramic element 12. The

reason that it is important to avoid such overlapping is that it canresult in the presence of a small gas gap, such as shown at 50, betweenthe flange 43a and the ceramic periphery 14. Such a gap is susceptibleto ionization, and this could trigger a harmful breakdown, If a similargap should be present between any of our flanges 43 and the outerperiphery of end ring 18, there would be little or no tendency to ionizethe gap because it is located behind electrodes 15 in a region of verylow electric field.

Another significant feature of our spacer ring 40 is that it extendsradially inwardly substantially beyond the inner periphery of itsassociated end ring 18. This relationship maintains button 24 radiallyspaced from the inner periphery of the end ring. This is of importancebecause occasionally some adhesive is extruded out from the interfacebetween end ring 18 and conductive coating 15 during the bakingoperation, forming a small bead (such as shown at 54) at the innerperiphery of the end ring. if the button 24 is allowed to closelyapproach the inner periphery of the end ring, there is a chance that itwill rideon to this bead of adhesive, and this would interfere withproper contact being made between the button and the conductive coating15. v

To facilitate bonding of the spacer rings 40 to their associated endrings 18, a'groove 56 is provided in each spacer ring at the innerperiphery of flange 43 where this inner periphery of flange 43 and thebody portion of the ring intersect. This groove assuresthat material ofthe spacer ring which would otherwise be present in this region does notinterfere with proper seating of the spacer ring on the end ringsurface.

While we have shown and described a particular embodiment of ourinvention, it will be obvious to those skilled in the art that .variouschanges and modifications may be made without departing from ourinvention in its broader aspects; and we, therefore, intend herein tocover all such changes and modifications as fall within the true spiritand scope of our invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is: v

1. In a high voltage ceramic capacitor assembly:

a. a plurality of ceramic capacitorblocks, each having an outerperipheral surface, a pair of opposed substantially planar'surfacesextending transversely of said peripheral surface, and a conductivecoating adherently bonded to each of said planar faces,

b. a pair of end rings of electrical-insulating material for each ofsaid ceramic blocks, each end ring having a surface bonded to one ofsaid conductive coatings and abutting said coating about substantiallythe entire outer peripheral region of said coating,

. each of said conductive coatings having a peripheral edge extendingabout the entire periphery of said ceramic block in precisely alignedrelationship with the peripheral surface of the ceramic capacitormaterial and the end ring material thereadjacent,

d. a thin coating of insulating material covering the peripheral edge ofeach of said conductive coatings and the immediately adjacent peripheralsurface of the ceramic block adjacent said conductive coating,

e. each of said end rings being of a material that has substantially thesame coefficient of thermal expansion as the ceramic of its associatedcapacitor block,

f. at least one conductive button,

g. a tubular housing of electrical insulating material,

h. said capacitor blocks being disposed in stacked relationship withinsaid tubular housing, with a conductive button j. said spacing meanscomprising, for each ceramic block, a

spacer ring of electrical insulating material anchored to 1 one end ringof said ceramic block and extending radially outward beyond the outerperiphery of said ceramic block, all portions of said spacer ring thatare disposed radially outward of the periphery of said ceramic blockbeing located in a position axially spaced from each of said ceramicblocks.

2. The capacitor assembly of claim 1 in which each of said spacer ringsis adhesively bonded to the end ring thereadjacent.

3. The capacitor assembly of claim 1 in which: there is associated witheach ceramic block a second spacer ring of electric insulating materialanchored to the other end ring of said ceramic block and extendingradially outward beyond the outer periphery of said ceramic block, allportions of said second spacer ring that are disposed radially outwardof the periphery of said ceramic block being located in a positionaxially spaced from each of said ceramic blocks.

4. The capacitor assembly of claim 3 in which:

a. each of said conductive buttons is surrounded by an end ring and thespacer ring anchored to said end ring, and

b. each of said spacer rings extends radially inwardly beyond the innerperiphery of its associated end rings, thereby maintaining the buttonthat is surrounded by said spacer ring radially spaced from said endring.

5. The capacitor assembly of claim 1 in which:

a. each of said spacer rings comprises a radially extending body portionand a flange at the outer periphery of said body portion extendingtoward its associated ceramic capacitor block axially of said tubularhousing,

b. the distal end of said flange terminates along a line axially spacedfrom said associated ceramic block, said flange surrounding the end ringadjacent said spacer ring.

6. The ceramic capacitor assembly of claim 5 in which:

a. each of said spacer rings and the end ring anchored thereto havegenerally flat faces bonded to each other with the flange on said spacerring surrounding the associated end ring, and

b. each of said spacer rings includes a circumferential groove at theinner periphery of said flange in the region where said inner peripheryand said body portion intersect, thereby eliminating any material inthis region that could interfere with the proper seating of thegenerally flat faces of said end ring and said spacer ring.

7. The capacitor assembly of claim 1 in which:

a. each of said buttons is surrounded by an end ring and a spacer ringon one of said capacitor blocks,

b. said button has a shoulder thereon that is located at the outer sideof its associated spacer ring in a location to hold said spacer ring inplace on its associated end ring should the spacer ring move axiallywith respect to the end ring.

8. The capacitor assembly of claim 7 in which: each of said spacer ringsincludes a flange at its outer periphery that surrounds its associatedend ring, said flange having a length that is greater than the distancebetween said spacer ring and the shoulder on the associated button,whereby the shoulder limits axial movement of said spacer ring to suchan extent that said flange remains in a position around said end ringwhen the spacer ring engages said shoulder.

l i t i l

1. In a high voltage ceramic capacitor assembly: a. a plurality ofceramic capacitor blocks, each having an outer peripheral surface, apair of opposed substantially planar surfaces extending transversely ofsaid peripheral surface, and a conductive coating adherently bonded toeach of said planar faces, b. a pair of end rings of electricalinsulating material for each of said ceramic blocks, each end ringhaving a surface bonded to one of said conductive coatings and abuttingsaid coating about substantially the entire outer peripheral region ofsaid coating, c. each of said conductive coatings having a peripheraledge extending about the entire periphery of said ceramic block inprecisely aligned relationship with the peripheral surface of theceramic capacitor material and the end ring material thereadjacent, d. athin coating of insulating material covering the peripheral edge of eachof said conductive coatings and the immediately adjacent peripheralsurface of the ceramic block adjacent said conductive coating, e. eachof said end rings being of a material that has substantially the samecoefficient of thermal expansion as the ceramic of its associatedcapacitor block, f. at least one conductive button, g. a tubular housingof electrical insulating material, h. said capacitor blocks beingdisposed in stacked relationship within said tubular housing, with aconductive button located between each adjacent pair of said capacitorblocks and contacting the conductive coatings thereon for connectingsaid blocks electrically in series, i. spacing means for maintaining theperipheries of said ceramic capacitor blocks radially spaced from saidinsulating housing by a predetermined minimum amount sufficient toeffectively prevent ionization in the gaps between said peripheries andthe tubular housing, j. said spacing means comprising, for each ceramicblock, a spacer ring of electrical insulating material anchored to oneend ring of said ceramic block and extending radially outward beyond theouter periphery of said ceramic block, all portions of said spacer ringthat are disposed radially outward of the periphery of said ceramicblock being located in a position axially spaced from each of saidceramic blocks.
 2. The capacitor assembly of claim 1 in which each ofsaid spacer rings is adhesively bonded to the end ring thereadjacent. 3.The capacitor assembly of claim 1 in which: there is associated witheach ceramic block a second spacer ring of electric insulating materialanchored to the other end ring of said ceramic block and extendingradially outward beyond the outer periphery of said ceramic block, allportions of said second spacer ring that are disposed radially outwardof the periphery of said ceramic block being located in a positionaxially spaced from each of said ceramic blocks.
 4. The capacitorassembly of claim 3 in which: a. each of said conductive buttons issurrounded by an end ring and the spacer ring anchored to said end ring,and b. each of said spacer rings extends radially inwardly beyond theinner periphery of its associated end rings, thereby maintaining thebutton that is surrounded by said spacer ring radially spaced from saidend ring.
 5. The capacitor assembly of claim 1 in which: a. each of saidspacer rings comprises a radially extending body portion and a flange atthe outer periphery oF said body portion extending toward its associatedceramic capacitor block axially of said tubular housing, b. the distalend of said flange terminates along a line axially spaced from saidassociated ceramic block, said flange surrounding the end ring adjacentsaid spacer ring.
 6. The ceramic capacitor assembly of claim 5 in which:a. each of said spacer rings and the end ring anchored thereto havegenerally flat faces bonded to each other, with the flange on saidspacer ring surrounding the associated end ring, and b. each of saidspacer rings includes a circumferential groove at the inner periphery ofsaid flange in the region where said inner periphery and said bodyportion intersect, thereby eliminating any material in this region thatcould interfere with the proper seating of the generally flat faces ofsaid end ring and said spacer ring.
 7. The capacitor assembly of claim 1in which: a. each of said buttons is surrounded by an end ring and aspacer ring on one of said capacitor blocks, b. said button has ashoulder thereon that is located at the outer side of its associatedspacer ring in a location to hold said spacer ring in place on itsassociated end ring should the spacer ring move axially with respect tothe end ring.
 8. The capacitor assembly of claim 7 in which: each ofsaid spacer rings includes a flange at its outer periphery thatsurrounds its associated end ring, said flange having a length that isgreater than the distance between said spacer ring and the shoulder onthe associated button, whereby the shoulder limits axial movement ofsaid spacer ring to such an extent that said flange remains in aposition around said end ring when the spacer ring engages saidshoulder.